No. i.] MORPHOLOGY OF THE PETROSAL BONE. 23 



The basisphenoidal part of the sphenoid bone of man forms 

 the floor and the anterior and posterior walls of the pituitary 

 fossa. The corresponding parts of the skull of Amia are occu- 

 pied by that part of the eye-muscle canal that lies immediately 

 below the membranous sac that forms the pituitary fossa. The 

 floor of this part of the canal is formed by a median portion 

 of the dorsal surface of the parasphenoid, and by carti- 

 laginous parts of the primordial cranium that are covered 

 externally by the lateral portions of the parasphenoid. In tel- 

 eosts the corresponding part of the skull is formed by the 

 petrosals and parasphenoid, the petrosals invading and repla- 

 cing almost entirely the cartilaginous parts of Amia. The 

 teleostean basisphenoid lies in front of this region, in the place 

 occupied by the presphenoid bone in man. 



The parasphenoid of fishes and Amphibia is said by Huxley 

 (No. 14, p. 27) to replace functionally the basisphenoid and 

 presphenoid of higher vertebrates, and to become, in these 

 latter animals, confounded with the basisphenoid. Parker 

 states more definitely (No. 20, p. 138) that "in the bird, the 

 basisphenoid borrows its ossifying center at first from the para- 

 sphenoid." 



The mammalian basisphenoid bone is thus probably not 

 developed in fishes because of the earlier development, in con- 

 nection with the dentition of these latter animals, of a large 

 parasphenoid bone which functionally replaces it. Whether 

 the parasphenoid is developed directly from the plates that 

 bear the teeth, by their fusion, as Sagemehl concludes (No. 27, 

 pp. 1 86, 199), or independently of those plates, from the con- 

 nective tissues underlying them, as Walther concludes (No. 33, 

 pp. 67, 78), is evidently unimportant in this connection, 

 excepting as the origin of the bone might determine or affect 

 its ultimate more or less complete incorporation in the skull. 



If then, in general summary, it be assumed that in some 

 animal presenting in the sphenoidal region of its skull the 

 features characteristic of Amia, the teeth that give origin, 

 either directly or indirectly, to the parasphenoid bone no 

 longer being needed, the bone itself begins to be replaced 

 or absorbed by a primary ossification, the basisphenoid; that 



